Perlitic grey iron for brake components

ABSTRACT

An iron alloy material for brake components comprising substantially perlitic grey iron alloyed with further elements comprising: carbon present in an amount of 3.50-3.70% by weight; vanadium present in an amount of 0.05-0.1% by weight; molybdenum present in an amount of 0.20-0.30% by weight; silicon present in an amount of 1.90-2.0% by weight; manganese present in an amount of 0.55-0.70% by weight; phosphor present in a maximum amount of 0.10% by weight; sulphur present in an amount of 0.08-0.12% by weight; chromium present in an amount of 0.20-0.30% by weight; and copper present in an amount of 0.20-0.30% by weight.

FIELD OF THE INVENTION

The present invention relates to an iron alloy material comprising greyiron alloyed with further elements. In particular the invention relatesto high-carbon-content grey iron components for braking systems whichundergo wear during use, such as brake drums or brake discs for example.

BACKGROUND TO THE INVENTION

Due to the use of brake shoes and brake pads which are free of asbestosdue to the health risks associated therewith, a significant problem hasdeveloped with regard to the useful life of the metallic componentsagainst which the shoes or pads bear. In heavy or high-speed vehicleapplications where the thermal and normal stresses are high, thisproblem is accentuated. Whereas the previous asbestos brake materialsfor example would result in a brake drum life in a heavy vehicle of some1×10⁶ km, the new asbestos-free brake materials might reduce the life,in certain cases, down to as low as 3.4×10⁵ km.

Further problems associated with brake drums and brake discs are brakesqueal, brake judder and brake fade. Brake squeal is a noise which has afrequency of between about 600 Hz and 2000 Hz in drum brake applicationsand is particularly unpleasant for the vehicle driver as well as othersin the audible area of the noise. This noise has been found not only tobe related to the brake geometry but also to the materials of the brakedrum/disc as well as the brake lining and has been found to be moreintense with presently used friction lining materials.

Brake judder is a low frequency vibration of about 7 to 8 Hz and occursdue to the brake drum or brake disc being out of round, which is oftenan effect caused by built-in deformations from the process of wheelmounting and high temperature resultant thermal stress, which in the endmay result in increased out-of-roundness in the brake component.

Brake temperature fade is a reduced braking effect caused, similar tobrake judder, by increase in temperature. However, brake fade is due, indrum brakes for example, to the diameter of the drum increasing at acertain rate whilst the diameter of the shoe does not increase at asimilar rate. The quality of the lining material also has an influence.

Several attempts have been made by others to overcome one or more of theabove-mentioned problems. For example U.S. Pat. No. 4 948 437 and U.S.Pat. No. 4 961 791 disclose certain grey iron alloy compositions andspecific subsequent heat treatments that have, as a combined process,been used to increase the tensile strength of the alloy, required afterhaving raised the carbon content in the material to increase theconductivity and thereby reduce brake fade and judder. Clearly the useof a subsequent heat treatment adds to the cost and time of manufacture.Additionally the resultant alloy requires further improvement withrespect to the aforementioned braking-associated problems.

A further array of possible alloy materials for disc brake applicationsis known from JP laid-open application no. 90-138438 which discloses adisc brake rotor made from an alloy developed to have high thermalresistance, high wear resistance and reduced brake squeal. Whilst thedisc brake rotor according to that application may have certainadvantages depending on the exact alloy composition chosen, the rangesgiven, such as for example the range of weight percentages for carbon(3.5 to 4.0%), for vanadium (0.02% to 0.35%), for copper (0.2 to 2%) andfor molybdenum (0.4 to 1.2%), are very large indeed and cannot be saidto give optimal results. Should any particular values inside these largeranges give an improved result, it is not stated what such restrictedranges or values might be. Indeed, the properties of the alloy obtainedvary to a very large extent between the ends of the disclosed ranges.

The main object of the invention is thus to provide an alloy which isoptimised for applications where the metal component is subject to wearby contact with a further component moving relative thereto.

In particular this object is directed to a brake drum or to a brake discfor automotive applications requiring optimised thermal resistance,squeal resistance and wear resistance having regard to the associatedbrake lining, with respect to which it is relatively movable (i.e. therelative movement being caused by the disc or drum rotation compared tothe stationary friction linings of the pads or shoes). At the same time,the component must not be too hard since this would make machining moredifficult, longer and thus more expensive.

A further object of the invention is to avoid the need for anysubsequent heat treatment process.

SUMMARY OF THE INVENTION

The object of the invention is solved by an alloy which has the featuresdefined in appended claim 1. Preferred features of the invention aredefined in the dependent claims.

One aspect of the present invention comprises:

carbon present in an amount of 3.50-3.70% by weight;

vanadium present in an amount of 0.05-0.1% by weight;

molybdenum present in an amount of 0.20-0.30% by weight;

silicon present in an amount of 1.90-2.0% by weight;

manganese present in an amount of 0.55-0.70% by weight;

phosphor present in a maximum amount of 0.10% by weight;

sulphur present in an amount of 0.08-0.12% by weight;

chromium present in an amount of 0.20-0.30% by weight; and

copper present in an amount of 0.20-0.30% by weight.

In a preferred embodiment of the invention a complete range of allvalues of elements of the alloy required for optimum performance hasbeen defined.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to a preferredembodiment thereof and with reference to the accompanying drawings inwhich,

FIG. 1 shows a sectional view of a brake drum incorporating the alloy ofthe invention, and

FIG. 2 shows a perspective view, in part section, of a brake discincorporating the alloy of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In the following description all percentages of the elements of thealloy are given in terms of weight. For example 4.0 to 4.2% means 4.0 to4.2% by weight.

The alloy of the invention is basically a high-carbon-content grey ironalloy which has an equivalent carbon content of between 4.0 and 4.2%.The actual quantity of carbon has however been reduced to between 3.5and 3.7% in the alloy of the invention.

The quantity of carbon added effects the E-modulus (modulus ofelasticity) of the alloy such that the higher the amount of carbon is,the lower the E-modulus obtained. With a lowered E-modulus the effect isobtained that the high temperature mechanical properties of the materialare increased. Additionally certain sound frequencies are damped. Forexample, a lowered E-modulus gives reduced force variations duringbraking for any given degree of out-of-roundness and thus the risk ofbrake judder can be reduced.

The heat conductivity of the iron is increased by the high content ofcarbon as is the heat capacity of the material, both of which areclearly desirable from a thermal fatigue point of view. This is the casesince the increased heat conductivity gives a faster temperaturedispersion in the rotational direction of the component which reducesthermal out-of-roundness effects and thus similarly reduces judder.

Furthermore carbon in graphite form will be present in the frictioninterface between the brake lining and the brake disc or drum which willassist heat dissipation directly due to the high heat conductivity ofgraphite which reduces the temperature gradient.

The graphite itself will mainly have the form of "graphite 1 A 3" asmeasured according to the ASTM standard norms.

The thermal resistance characteristics are also better in this materialdue to the fact that the low modulus of elasticity results in lowerstresses for a given temperature change.

However the carbon must not be present in a higher amount since thiswould be detrimental to strength.

The matrix should be as close as possible to being 100% perlitic sincesmall percentages of ferrite (e.g. of the order of 5%) would severelydecrease wear resistance and the mechanical strength would be impaired.Since the brake component is clearly a prime safety item of a vehiclethe obtaining of a near 100% perlitic structure is very important.

Vanadium is present in the alloy in an amount of 0.05-0.1%. As theamount of vanadium approaches 0.1% and above, the alloy becomes moreexpensive due to the vanadium content as well as being much harder andthus more expensive to machine (due to wear on the cutting tool(s)).However, if the amount is too low, wear resistance is reduced in thefinal alloy according to an adverse curvilinear relationship. Thus theamount of vanadium must be very carefully balanced to achieve thedesired effect but without giving outweighing disadvantages.Consequently the amount of vanadium has been found to lie most suitablyin a range of 0.07 to 0.08% to achieve the best balance of the requiredproperties.

In order to achieve the expected results, molybdenum is present in anamount of 0.20-0.30% and the very best results are obtained in an amountof 0.25%. This element has the function of stabilising the perlite andthus increasing the mechanical strength of the alloy. At highertemperatures the molybdenum also serves to stabilise the pearlite andthus has the effect of increasing the thermal fatigue resistancecharacteristics.

If the amount of molybdenum is greater than 0.30% (e.g. about 0.40% ormore) the material becomes very difficult to machine and the price percomponent thus becomes undesirably high.

In order to provide a complete solution for all the aforementionedproblems, whilst providing an easily machinable component, the alloyalso has certain additional elements as explained below.

In order that the carbon content should be equivalent to 4.0 to 4.2 (toobtain a eutectic precipitation of graphite and austenite respectively),phosphor and carbon are added. Silicon is added in an amount of between1.9 to 2.1% such that a grey solidification is achieved. Additionally,silicon sets the ferrite and thus improves the mechanical strength ofthe matrix, although the amount of silicon should be restricted sincethis will reduce the heat conductivity of the alloy. However since theamount of phosphor should also be held as low as possible to avoidundesirable precipitations of phosphides, the addition of silicon in theprescribed amount is important to the end result.

Manganese is present in the alloy in an amount of between 0.55 and 0.70%and has the effect of stabilising the pearlite and thus improvingmechanical strength.

Phosphor is present in a maximum amount of 0.1% and acts as a graphitestabiliser. However the content should be kept as low as possible inorder to reduce the proportion of phosphide eutectic.

Sulphur should be present in an amount of between 0.08 to 0.12%.

Chromium is present in an amount of between 0.2 and 0.3% in order toincrease the durability and wear resistance of the alloy, not only bystabilising the pearlite but also by building secondary carbides at thecell borders. These carbides are precipitated during solidification dueto the fact that chromium segregates in the matrix. The amount ofchromium which does not form carbides will have been dissolved in thepearlite's ferrite.

In order to avoid an undesirable amount of carbide the amount ofchromium has to be "matched" with the amount of vanadium present in thealloy, since too high an amount will lead to the risk of primary carbideprecipitation or to the precipitation of larger secondary carbides.

Copper is present since it is a large contributor to the formation ofpearlite and is also a pearlite stabiliser (which increases mechanicalstrength). Copper also reduces the formation temperature in the solidphase when the pearlite is being formed, thus producing a finerstructure which is important for hardness and breaking strength. Copperfurther serves to set the pearlite's ferrite and balances the chromiumquantity whilst also opposing the formation of primary solidificationcarbides.

The remainder of the alloy material is constituted by the by grey ironand any other minor impurities.

Whilst the alloy can be produced to give the desired results within thenarrow ranges or amounts defined above for the various elements, testshave been successfully carried out on brake drum prototypes using analloy having a weight content of carbon of 3.65-3.70%, silicon of 2.0%,copper of 0.25%, chromium of 0.25%, molybdenum of 0.25% and vanadium of0.07-0.08%, the remaining materials being as previously stated. Such analloy used in a brake drum results in an alloy having a hardness of atleast 170 according to the HB (5/750) test, i.e. a Brinell hardnessusing a 5 mm steel ball and 750 kg load.

Tests on a brake drum made from the alloy material of the invention asspecified in the preceding paragraph have shown that the aforementionedprecise combination of elements in the alloy produces significantimprovements in all aspects simultaneously.

The method of manufacturing the components requires that the inoculationand casting temperature are adapted to the alloy. Thus the inoculationshould be relatively high in order that primary carbides are avoided.Consequently, as an example, an amount of approximately 0.20% FeSi mightbe used as the inoculation additive. Similarly, the casting temperatureshould be lowered somewhat (suitably by some 20 to 30° C. compared withstandard iron) in order to obtain a suitable over-temperature in themelt.

The smelting process is essentially the same as for most iron alloys andthus need not be described in great detail as this will be clear to theskilled man. The base molten iron component, containing silicon,manganese, phosphor and sulphur is produced in a cupola furnace afterwhich it is transferred to a buffer oven. When the molten iron is inthis form the additives are added to the melt in a transport ladle. Thecarbon is added in the form of graphite powder and the rest of the alloyadditives are added in the form of FeCr, FeMo, and FeV. Inoculationmaterial may be added at this stage and the silicon level may also beadjusted to give the correct equivalent carbon content. Copper is addedin any suitable form of metal particles (e.g. filings) and a finaladjustment of the required chemical composition is made in the castingoven prior to casting. Further inoculation material (e.g. FeSi) is thenadded to the stream of molten metal during passage from the casting ovento the mould.

After machining the casting to the exact dimensions of the drum or disc,no heat treatment is required.

FIG. 1 shows a sectional view of a preferred design of brake drum 1 ofthis invention for a heavy vehicle (e.g. a vehicle having four axles oreight axles with two or four wheels per axle for instance). The drumwill be substantially symmetrical about centreline X--X and thus onlyone half of the drum is shown. Numeral 2 denotes the brake lining of abrake shoe which is applied to the inside of the drum to effect braking.Part of a dirt cover 3 is also shown in cross section.

The brake drum of such a heavy vehicle is particularly suitable for theapplication of the alloy of this invention since heavy vehicles of thistype are normally required to do high mileage during their lifetime,whereas existing brake drums or discs may still be suitable for lighterautomotive use (e.g. cars, vans or motorcycles) where the likely mileageis significantly lower. However the alloy of this invention may alsofind application in these vehicles as well.

Brake drums for modern large or heavy vehicles are quite deep and willhave an inner diameter of 300 mm or more. Typically the inner diameterof the drums will be somewhere between 340 mm to 430 mm, with weightvarying normally between about 35 kg and 85 kg. With the alloy of theinvention however, since the wear resistance will be increased sosignificantly, the depth or the diameter of the drum (and thus also thepads or shoes) may be reduced whilst still obtaining sufficient wearlife from the drum. This is clearly a significant advantage for theautomotive industry and the end-user since not only is cost saved, butthe weight (which is furthermore unsprung weight) is reduced along withall the ensuing advantages of better performance, reduced fuelconsumption and better handling etc.

The alloy of the invention may also find application in cars or othersmaller vehicles where the drum inner diameter or disc diameter is muchsmaller than in heavy vehicles.

FIG. 2 shows an example of a brake disc 4 which can be manufactured fromthe alloy of this invention. Similar reductions and savings to thosedescribed above with respect to drum brakes may also be made.

The exact form or type of brake drum or disc may of course vary widelyfrom those depicted depending on the particular application.

By the use of the alloy according to the invention, no subsequent heattreatment of the machined drum or disc is required at all to achieve thedesired results. Clearly this represents a significant cost and timereducing factor in comparison to many prior art solutions. However iffor some other reason a subsequent heat treatment is applied, theimproved braking properties of the material are maintained.

Using the alloy of this invention with present asbestos-free frictionlining material (e.g. ABEX 929 or Mintexdon 7115 friction materials),the improvements in wear resistance obtained may be of the order of 200%or more which is a tremendously significant achievement. Even when thedrum is used under extreme driving/braking conditions an increase ofbetween 50 and 100% can be obtained. Even this increase is still atremendous increase in drum life. Thus, it will be appreciated that bythe careful study and intricate selection of the required alloy elementsthe inventors of the present alloy have arrived at an optimised brakebody material which has distinct and significant mechanical and thermaladvantages compared to the prior art materials used.

Tests have also shown that by using a drum or disc in accordance withthe invention in combination with present day conventional brake pads orshoes, an increase in friction lining life of between 5 and 10% can alsobe obtained. This advantage may be used for example to increase servicereplacement intervals and thus generally lower cost.

The invention is not restricted to the embodiments described above butmay be varied widely within the scope of the appended claims. Forexample, whilst the alloy has been described with respect to rotatingbrake components for which it has been specifically designed, the alloymay also be used in other fields, for example clutches of differingtypes or other applications where frictional contact between the alloyand a further lining surface occurs.

We claim:
 1. An iron alloy material comprising:carbon present in anamount of 3.50-3.70% by weight; vanadium present in an amount of0.05-0.1% by weight; molybdenum present in an amount of 0.20-0.30% byweight; silicon present in an amount of 1.90-2.0% by weight; manganesepresent in an amount of 0.55-0.70% by weight; phosphor present in amaximum amount of 0.10% by weight; sulphur present in an amount of0.08-0.12% by weight; chromium present in an amount of 0.20-0.30% byweight; and copper present in an amount of 0.20-0.30% by weight; withthe balance being substantially perlitic grey iron.
 2. The iron alloymaterial as claimed in claim 1 wherein said vanadium is present in anamount of 0.07-0.08% by weight.
 3. The iron alloy material as claimed inclaim 2 wherein said carbon is present in an amount of 3.65 to 3.7% byweight, said copper is present in an amount of 0.25% by weight, saidchromium is present in an amount of 0.25% by weight, said molybdenum ispresent in an amount of 0.25% by weight, and said silicon is present inan amount of 2.0% be weight.
 4. The iron alloy material as claimed inclaim 1 wherein said vanadium is present in an amount of 0.075% byweight and said molybdenum is present in an amount of 0.25% by weight.5. A brake drum manufactured from an alloy material according toclaim
 1. 6. The brake drum as claimed in claim 5 wherein said brake drumis an automotive brake drum.
 7. The brake drum as claimed in claim 5wherein said brake drum has a diameter equal to or greater than 300 mm.8. A brake disc manufactured from the alloy material according to claim1.